1. Field of the Invention
The present invention is related to an operational amplifier capable of compensating offset voltage, and more particularly, to an operational amplifier compensating offset voltage through adjusting currents between an input stage circuit and an output stage circuit of an operational amplifier.
2. Description of the Prior Art
An operational amplifier is an important component of all kinds of electronic devices, and is widely used in home appliances, industrial fields, scientific instruments, etc. Circuit designers usually use operational amplifiers for different operations, such as buffers, filters, analog to digital converters, etc.
An ideal operational amplifier has the following characters: infinite input impedance, zero output impedance, infinite open loop gain, infinite common mode rejection ratio, infinite frequency bandwidth. However, due to limitations of semiconductor processes and integrated circuit techniques, there are difficulties in realizing an ideal operational amplifier. In order to present properties of a real operational amplifier, an offset voltage is used for representing the non-ideal effects in the prior art.
Please refer to FIG. 1, which illustrates a schematic diagram of a prior art operational amplifier 10. The operational amplifier 10 comprises an input stage circuit 100, an output stage circuit 102, and an equivalent voltage source 104. The operational amplifier 10 receives differential signals from a positive input end Vp and a negative input end Vn, and outputs an amplified result from an output end Vo. The equivalent voltage source 104 represents non-ideal effects of the operational amplifier 10 (which does not exist in the actual circuit), a generated voltage Vos represents the voltage difference between the output end Vo and the positive input end Vp when the output end Vo is coupled to the negative input end Vn (forming a unit gain feedback structure), namely an offset voltage of the operational amplifier 10. Many reasons cause offset voltage, such as physical properties of semiconductors, flaws during process, component mismatches, etc.
Please refer to FIG. 2, which illustrates a schematic diagram of the input stage circuit 100 shown in FIG. 1. The input stage circuit 100 generates currents Id1 and Id2 to the output stage circuit 102 according to the positive input end voltage Vp and the negative input end voltage Vn, and includes input transistors P1, P2 and a current source 200. The input transistors P1 and P2 are both p-type metal oxide semiconductor (MOS) transistors forming a common-source differential pair with a common mode voltage Vc. Under ideal conditions, properties such as channel lengths or channel widths of the input transistors P1 and P2 are identical, so a current Ib generated by the current source 200 will drift into the sources of the input transistors P1 and P2 evenly, making source-to-gate voltages of the input transistors P1 and P2 identical, which means that the voltage Vos equals 0, and the currents Id1 and Id2 are identical. However, in practice, the input transistors P1 and P2 do not match perfectly due to processing properties, so that threshold voltages of the input transistors P1 and P2 are not the same. In such situation, the source-to-gate voltages of the input transistors P1 and P2 are not identical, so that the offset voltage Vos does not equal 0, and the currents Id1 and Id2 are not identical.
Briefly speaking, due to physical properties of semiconductors, flaws during process, component mismatches, etc, the offset voltage of the prior art operational amplifier does not equal 0, and has an effect on the performance. Therefore, decreasing the operational amplifier offset voltage effect became one of the tasks in this art.